Aaron L. Berger

Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal

Focused denudation and mid-crustal flow are coupled in many active tectonic settings, including the Himalaya, where exhumation of mid-crustal rocks accommodated by thrust faults and low-angle detachment systems during crustal shortening is well documented. New structural and (U-Th)/He apatite data from the Mount Everest region demonstrate that the trans-Himalayan Ama Drime Massif has been exhumed at a minimum rate of ~1 mm/yr between 1.5 and 3.0 Ma during orogen-parallel extension. The Ama Drime Massif offsets the South Tibetan detachment system, and therefore the South Tibetan detachment system is no longer capable of accommodating south-directed mid-crustal flow or coupling it with focused denudation. Previous investigations interpreted the NNE-SSW–striking shear zone on the west side of the Ama Drime Massif as the Main Central thrust zone; however, our data show that the Ama Drime Massif is bounded on either side by 100–300-m-thick normal-sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset Quaternary deposits and record active orogen-parallel extension. When combined with existing data, these results suggest that the Ama Drime Massif was exhumed during orogen-parallel extension that was enhanced by, or potentially coupled with, denudation in the trans-Himalayan Arun River gorge. This model provides important insights into the mechanisms that exhumed trans-Himalayan antiformal structures during orogen-parallel extension along the southern margin of the Tibetan Plateau.

Denudation and deformation in a glaciated orogenic wedge: The St. Elias orogen, Alaska

Apatite (U-Th)/He dating from the St. Elias orogen in southern Alaska illustrates a potential association between long-term denudation and glacier sliding. Cooling ages as young as 0.4 Ma (exhumation ~4–5 mm/yr) are concentrated in a narrow band near the glacier equilibrium line altitude (ELA) front, where mean Quaternary ELA intersects the windward flank of the orogen. This band of denudation is not correlated with individual faults, structural trends, or known concentrations of precipitation, and we propose that it is produced by focused glacier sliding at or near the ELA front. This implies that long-term glacial erosion is a maximum at ELA, which corroborates model predictions that glaciers and climate can control the pattern of crust removal from orogens. Denudation rates do not covary with fluctuations in glacier size along the ELA front, suggesting that small glaciers are capable of keeping pace with incision by larger ones and tectonic rock uplift. Ice discharge may thus play a critical, but complex, role in excavating glaciated orogens.

Neotectonics of the Yakutat Collision: Changes in Deformation Driven by Mass Redistribution

The most recent period of orogenesis in southern Alaska began in the late Neogene with the collision of the Yakutat microplate, which is partially accreted to and partially subducted beneath the Alaskan margin at the easternmost extent of the Aleutian Trench. Neotectonic studies suggest significant spatial and kinematic variation in active deformation during the collision of the Yakutat microplate. The Saint Elias orogen experienced a widespread structural reorganization in the Quaternary with oblique convergence partitioned onto an en echelon thrust array. The new tectonic configuration also includes the continuing development of an incipient indentor comer, significant retrothrust motion, and shifting deformation fronts. Reorganization is temporally linked to intense glacial erosion in the core of orogen and rapid sedimentation in offshore depocenters during the Pleistocene. We propose that mass redistribution and modification of orogenic topography played an integral role in the structural and tectonic evolution of the present system. Currently, the spatial deformation front (outboard limit of deformation) and active deformation front are not the same, suggesting that deformation swept through the landscape through time, presumably as a result of glaciation, tectonic adjustment, or both. A more complete picture of the complex response of near-surface deformation to topographic disruption should improve seismic hazard assessments.

Neogene Exhumation of the Tordrillo Mountains, Alaska, and Correlations With Denali (Mount Mckinley)

To better understand the timing of mountain building of the western Alaska Range in the Tordrillo Mountains, a preliminary suite of 10 samples from Paleocene granite was collected for apatite fission track (AFT) and apatite (U-Th)/He (AHe) thermochronology from elevations between 295 and 3231 m. We obtained a zircon fission track age on one sample of 59 Ma, which is close to the 60 Ma age of crystallization. One AFT sample shows evidence of rapid cooling at 35 Ma, and the others had an initial phase of rapid cooling at ∼23 Ma, followed by a period of relative stability until another phase of rapid cooling that started at ∼6 Ma. AHe data from the highest two samples also indicate rapid cooling at 6-8 Ma. The remainder of the samples have AHe ages older than the AFT ages and in one case older than the concordant U/Pb zircon date, which may be due to zonation of the apatite crystals. Comparison of the Tordrillos AFT data with previously published AFT data from the Denali area indicates both experienced rapid cooling starting at ∼6 Ma. The timing of exhumation also represents surface uplift because voluminous Pliocene sediments of the Sterling Formation fill the adjacent Susitna and Cook Inlet basins. We infer that synchronous uplift of the central and western Alaska Range occurred as a result of counterclockwise rotation of southern Alaska, south of the Denali fault, as a far-field effect of the Yakutat microplate collision and flat-slab subduction.

Structural relationships in the eastern syntaxis of the St. Elias orogen, Alaska

The eastern syntaxis in the St. Elias orogen (Alaska, USA) is one of the most complex and least understood regions within the southern Alaska coastal mountain belt. The syntaxis contains many features unique to the orogen that are essential to understanding the structural architecture and tectonic history of the collision between North America and the allochthonous Yakutat microplate. The eastern syntaxis contains the transition from transpressional structures associated with the Queen Charlotte–Fairweather fault system in the east to the Yakataga fold-and-thrust belt (YFTB) to the west. Throughout the eastern syntaxis, a prominent unconformity at the base of the synorogenic Yakataga Formation records an erosional event related to the development of the YFTB. Strain accumulations in the eastern YFTB predate the deposition of the Yakataga Formation, extending estimates for the early development of the St. Elias orogen. Structural and stratigraphic relationships in the eastern syntaxis suggest that forethrusts associated with the transpressional system shut down and were overprinted by fold-and-thrust structures in the Early to latest Miocene. Basement in the eastern syntaxis consists of the Yakutat Group, part of the Chugach accretionary complex, which is carried by numerous low-angle thrust faults in the eastern syntaxis. Exposures of basement and fault patterns within the syntaxis have implications for tectonic reconstructions of the Yakutat microplate and the geodynamics of the orogen.

Exhumation in the Chugach‐Kenai Mountain Belt Above the Aleutian Subduction Zone, Southern Alaska

Convergent deformation systems in continental backstops are a common component of well-coupled subduction zones worldwide. The Aleutian megathrust offshore southern Alaska has many attributes in common with convergent subduction zones observed elsewhere, implying that significant permanent shortening could occur within the continental hanging wall. A continuous belt of rugged mountains occurs along the forearc of this subduction zone, from the Kenai Peninsula to the eastern Chugach Mountains near the Copper River, providing further evidence of active shortening. To test for long-term shortening and associated rock uplift along this forearc, we have analyzed bedrock samples from the Chugach and Kenai Mountains using low-temperature thermochronometry. Fourteen new apatite (U-Th)/He ages from this area are older than expected based on the rugged topography, and imply minimal exhumation in the late Cenozoic in response to shortening within the forearc. Ages on the leeward side of this mountain belt are ∼20―40 Ma and imply an average exhumation rate of ∼0.1 mm/a or less. Ages are younger along the coast (∼12―18 Ma), implying slightly more rapid exhumation or deeper incision relative to mean elevation, perhaps due to greater precipitation along the windward flank of the range. Younger ages (∼5 Ma) occur in the southeast near Cordova, and may result from local deformation associated with colliding and underthrusting of the Yakutat microplate. However, all of these cooling ages are older than ages from the Yakutat collision zone and along the transpressional Fairweather fault to the east, indicating that far less exhumation occurs in the backstop above the subduction zone. Based on these older ages and a mass balance interpretation of exhumation, we estimate that the long-term shortening in the forearc mountains above the Aleutian megathrust is less than 1 mm/a.